Arc welding carries out welding by generating arc between a welding wire and a welding base material. A consumable electrode type arc welding apparatus (hereinafter, referred to as “a welding apparatus”) predicts that a short circuit is opened and reduces welding current just before short circuit is opened in order to suppress the generation of spatter. This is a so-called neck detection control. Then, arc force is suppressed by reducing welding current at the moment at which arc is generated again.
In a conventional welding apparatus, when a short circuit occurs, a control circuit for controlling a welding output increases an electric current in accordance with a predetermined tilt in order to open the short circuit. Then, in accordance with this increase of electric current, a wire in a short-circuit part is melted and the molten metal is shifted to the side of a base material, so that a constricted portion (a so-called neck, which is referred to as “neck,” hereinafter) is formed between the welding wire and the molten portion at the side of the base material. Since the cross-sectional area at this neck portion becomes small, the resistance value is increased. Therefore, when neck is formed, although the increase of electric current is kept constant by short-circuit control, a change amount of welding voltage is increased. Then, by detecting the change amount of voltage, the formation of neck is detected, and the process is proceeded to neck detection control.
Herein, a conventional welding apparatus is provided with a switching element for switching rectified AC power supply in order to obtain an output suitable for welding as mentioned below. Then, under the effect of this switching element, ripple voltage synchronous with switching of the switching element is superimposed on the welding voltage. This ripple voltage may lower the neck detection probability. Therefore, Japanese Patent Unexamined Publication No. H10-180443 discloses removal of noise such as ripple voltage.
FIG. 8 shows a schematic configuration of a conventional welding apparatus. The welding apparatus includes first rectifying element 81 for rectifying AC power supply, first switching element 82 for switching the output from first rectifying element 81, main transformer 83 supplying welding load with electric power and being provided with secondary side auxiliary winding, second rectifying element 84 for rectifying the output from main transformer 883, third rectifying element 85 for rectifying the output from the auxiliary winding of main transformer 83, second switching element 86 provided between second rectifying element 84 and an output terminal, current detector 87, voltage detector 88, differential amplifier circuit 89 for operating the difference between the output from voltage detector 88 and the output from third rectifying element 85, and welding control circuit 810 for controlling a welding output by controlling first switching element 82 and second switching element 86.
Next, an operation of the apparatus is described. Electric power supplied from an AC power supply is made into a direct current by first rectifying element 81. The direct current electric power is input into main transformer 83 as necessary electric power in accordance with a welding state by first switching element 82. A large current obtained from the output side of main transformer 83 is made into a direct current by second rectifying element 84, and supplied as an electric power to a welding load through second switching element 86. Welding current flowing to the welding load is detected by current detector 87 and fed back to the control circuit. Furthermore, welding voltage applied to the welding load is detected by voltage detector 88. Herein, the welding voltage detected by voltage detector 88 includes a ripple component generated by first switching element 82. Furthermore, the output from third rectifying element 85 includes a ripple component by first switching element 82. Thus, the difference between the output from third rectifying element 85 and the output from voltage detector 88 is operated by differential amplifier circuit 89, and thereby the ripple component included in the welding voltage can be removed. Thus, welding voltage from which the ripple component has been removed is input into welding control circuit 810 and used for accurate detection of a neck portion or control of welding output.
As mentioned above, a conventional welding apparatus can carry out control and the like of the welding output by removing the effect of a ripple component by a switching element constituting a welding apparatus itself. However, there has been a problem that other noises made by the welding apparatus itself or noises caused by the influence of the other equipment, and the like, could not be removed sufficiently. The noises caused by the influence of the other equipment include, for example, as shown in FIG. 9, noise caused by a switching operation for output control of one welding apparatus 1 and generated in welding voltage of the other welding apparatus 2 when weldings are carried out with respect to the same work by a plurality of welding apparatuses. In FIG. 9, references A, B and C indicate noises of voltage waveform of welding apparatus 2 caused by the switching operation of welding apparatus 1. Likewise, references D, E and F indicate noises of voltage waveform of welding apparatus 1 by the switching operation of welding apparatus 2. Then, in a case where noise cannot be removed, wrong detection of neck occurs. As a result, due to this wrong detection of neck, since detection control is carried out with respect to a neck that is not a neck by nature, it has been difficult to suppress the occurrence of spatter or to obtain an excellent welding result. Furthermore, depending upon the conditions such as noise, opening of short circuit may be detected wrongly.